Pad printers are used for printing on irregular surfaces. In some forms of pad printers the ink is pumped into an ink tray which is exposed to the atmosphere. However, since the ink is thinner-based its viscosity increases over time to an unacceptable level due to the evaporation of the thinner. One means that has been devised to overcome this problem is to provide a cup to contain the ink so that the ink is not exposed to the atmosphere. Cups that are made of nylon or similar plastics material are known, as are cups made of hardened steel. The former are unsatisfactory in that they tend to distort and so cause uneven application of ink. The latter are expensive to manufacture and have a tendency to damage the die plate.